We propose a meta-analysis based on individual patient data from the two largest randomized trials to date that evaluate the efficacy of medical treatment in delaying or preventing the onset of primary open angle glaucoma in ocular hypertensive individuals. The European Glaucoma Prevention Study (EGPS) randomized 1,077 participants and the Ocular Hypertension Treatment Study (OHTS) randomized 1,636 participants. OHTS and EGPS share similarities in study design and measurement protocols including centralized masked randomization, lOP measurement, corneal thickness measurement, ascertainment of POAG, and cause-specific attribution of abnormality. Baseline clinical characteristics of the EGPS and OHTS participants are also similar. The pooled EGPS and OHTS datasets will include data from 2,713 OH participants with a median follow-up of more than 5 years and over 245 POAG endpoints. The pooled sample will provide statistical power to test hypotheses that might not be possible with one dataset alone. Similarities in study design and baseline participant characteristics suggest that pooled analyses will be highly productive. This collaboration will not interfere with each study's ability to analyze its own dataset. The Specific Aims of the proposed meta-analyses are: 1. To improve the precision of risk coefficients for known risk factors for developing POAG such as age, lOP, cup/disc ratio and central corneal thickness and to assess whether other putative risk factors such as gender, high blood pressure, low blood pressure, heart disease and diabetes should be included in the risk model. 2. To determine the risk of developing POAG (hazard ratio) at different levels of lOP over follow-up in both the medication and observation/placebo groups adjusting for baseline covariates - i.e., what is the difference in prognosis if a patient on medication achieves an lOP of 18 versus 21 mm Hg. 3. To determine the predictive accuracy of transient visual field defects during follow-up for developing POAG. 4. To determine the predictive accuracy of baseline HRT indices or changes in these indices during follow-up for developing POAG. The Steering Committees, Full Investigative Groups and Data and Safety Monitoring Committees of the EGPS and OHTS have approved the collaborative analyses. A Joint Committee will be responsible for implementing the study, overseeing the analyses and preparing presentations and publications.